This invention relates to a apparatus for bipolar electrosurgery, and in particular to surgical apparatus and a cutting assembly for cutting living tissue using the principle of radio frequency (RF) electrosurgery.
Bipolar RF electrosurgery historically has only been used for coagulation. Cutting by RF electrosurgery is normally performed by monopolar devices. The difference between the two techniques is that a bipolar device has two active electrodes for use at the site of operation, whereas a monopolar device has a fixed return electrode (usually a plate positioned on the patient's thigh or back) and one active electrode.
The established means for providing electrosurgical cutting is by producing very high voltage levels, typically in excess of 300 volts rms to create RF arcing. The arcing causes cell destruction in the path of an electrode. To the knowledge of the applicants, there are no known devices available at present which are designed for "dry" bipolar RF electrosurgical cutting. However, a crude arrangement is known for use in "wet-field" surgery, in which two electrodes are dragged over the tissue to be cut in the presence of an irrigation solution. The purpose of irrigation of the tissue is to reduce the electrical resistance between the electrodes when they are first applied to the tissue so that an arc can be initiated. The principal factor impeding development of bipolar technology is that it is very difficult to direct the arc, which occurs between the two active electrodes, onto the patient. It is far easier to make the patient the return path for the RF energy as in monopolar electrosurgery.
Monopolar electrosurgery has two distinct disadvantages. Firstly, having only one active electrode means that cautery is not nearly as effective as with bipolar electrosurgery. A typical cautery requirement is to seal a blood vessel, and since a bipolar electrode arrangement creates a current path across the two active electrodes, the blood vessel is cauterised side-to-side, thereby minimising damage to adjacent tissue. In contrast, monopolar cautery, due to the remote position of the return electrode in another part of the patient's body, results in current passing along the tissue planes, which creates more damage. The other disadvantage, which is perhaps more important, is the effect of the patient connection of the return electrode. A considerable proportion of the RF energy used in monopolar electrosurgery is dissipated in the patient's body. The maximum output of a monopolar electrosurgical unit is typically 300 watts, and up to 200 watts may be dissipated elsewhere in the body. This can cause significant internal heating and concerns have been expressed about possible consequences. Another effect of using patient connection is that the patient does not assume the return or neutral electrode potential. As a result of the significant voltage drop across the torso, earthed components connected to the patient can create a sufficiently large current to cause RF burns outside the operating site. The most common cause of burns outside the operating site is the resistance between the return electrode and the patient. When this resistance increases, the localised dissipation increases, thus causing burns.
It is also known to perform electrosurgical cutting by means of a hot wire tool. Specifically, a metallic wire loop is heated using a low frequency alternating current, and the cutting action is purely by heat. In U.S. Pat. No. 4,089,336, the metallic wire loop is replaced by a blade having a loop of material which has a negative temperature coefficient of resistance so that power dissipation is concentrated in that portion of the loop which is cooled by application to the tissue to be cut. Hot-wire devices are, however, limited in their cutting speed by the maximum heat that can be generated without significantly reducing the stability and strength of the heated element.